Conventional polyurethanes have taken a prominent position in the world as an inexpensive material for use in such diverse applications as foam insulation, adhesives, structural foams, shoe sole and others. The properties of conventional polyurethanes that make these uses possible are its physical strength, low cost, ability to make very low density foam, chemical resistance, and thermal stability.
One of the variations of the product family is the manufacture of what are called reticulated foams. These foams are constructed such that the “windows” that separate the individual cells making up the foam structure are open and the material in the windows collapse into the “struts and beams”. Thus when fully cured and viewed in a microscope, all one sees is a matrix of “tinker-toy-like” rods connected to one another at the ends. One of the advantages of this structure is that it presents very low resistance to the flow of air or water. It is often used as a filter media due to their typically low density and corresponding low cost per unit volume. These foams are hydrophobic, i.e. they do not absorb water.
Hydrophilic polyurethanes, on the other hand, while being of similar chemistry, are used in applications where being compatible with water is the primary reason for their use. These uses include controlled delivery devices, chronic wound care dressings and agricultural media.
An advantage of hydrophilic polyurethanes over conventional reticulated polyurethanes is their ability to be formulated with active ingredients. Hydrophilic polyurethanes are conventionally made by the emulsification and curing of an aqueous phase with a hydrophilic polyurethane prepolymer. The aqueous phase may contain an active ingredient in which case the ingredient is dispersed in the matrix of the resultant foam. In part, it is this ability to incorporate a wide variety of components in the aqueous phase that makes this chemistry commercially attractive.
The essential difference between these two related chemistries is that the hydrophilic polyurethane is compatible with and absorbs water while the conventional polyurethanes are hydrophobic and are incompatible with water. While this hydrophilic nature gives hydrophilic polyurethane its unique applications, it also leads to certain deficiencies. Among these are low physical strength, poor cell size control, relatively high densities causing a relatively high cost per unit volume, and the fact that foam swells considerably upon absorption.